1. Field of the Invention
The present invention relates to an electronic still camera which permits purely electronic photographing of an object and, more particularly, to improvement in a recording unit of such an electronic still camera.
2. Description of the Prior Art
Recently an electronic still camera has been proposed which employs, in combination, an optoelectro transducer, such as a solid state image sensor or pickup tube, and a recording unit using an inexpensive and large-capacity magnetic tape as a recording medium. Such a camera is adapted to take a still picture of an object purely electronically and to record the picture on the magnetic tape so that the image may be reproduced through the use of a separately provided television system or printer. This type of camera is attracting attention as a future substitute for the existing optical camera which involves chemical treatment of the film for development.
In still photography, since high picture quality is generally required, an optoelectro transducer with a large number of picture elements is needed. Further, the number of quantized bits per picture element must also be large. Since the number of still pictures that can be taken per unit time (the frame speed) is small, unlike in the case of motion picture photography, the read rate of the optoelectro transducer may be low and the quantity of information that must be recorded per unit time can be reduced. A reduction in the quantity of information that must be recorded per unit time can be accomplished by employing a solid state image sensor equipped with the image storage function, for temporarily storing signals from the solid state image sensor in a buffer memory and then reading out the signals at a low speed. Therefore, the recording device for the electronic still camera of the abovesaid type may be, for instance, a magnetic recording device of audio grade, permitting miniaturization of the camera and a reduction of its cost and power dissipation. It is considered that such an electronic still camera would be a good match for conventional optical cameras.
It is noted that still photography is intended primarily for independently observing images of individual frames, it is not suitable to reproduce images of still pictures by using a motion picture image reproducing method according to which a plurality of closely related recorded images are reproduced in the same sequence as that for recording. Thus there is great necessity for quickly reproducing an optimum one of a number of frames or a plurality of frames obtained by multi-exposure photographing one after another. A magnetic tape, however, is essentially a sequential file, and is lacking in random accessibility. Therefore, its improvement is desired, but little attention has been paid to this problem in the past, so that conventional electronic still cameras have the disadvantage of poor random accessibility for playback.
The prior art electronic still cameras employ a system of recording image data on a magnetic tape while forming thereon a recording format during magnetic recording and, in this system, creates varying lengths of record areas, for recording each frame of information, record gaps and file gaps, used as a tape starting and stopping area, vary owing to the dispersion in the characteristics of a tape drive unit and so on. This leads to the shortcoming that a reproducing device cannot accurately recognize information or the position of data to be reproduced. Namely, when it is desired to retrieve an immediately preceding frame during reproducing, since the distance to its position is not available on the side of the reproducing device, the retrieval must always be started from the beginning of the tape or a specified file and, consequently, a long access time is needed. This defect is particularly marked in the case of using a magnetic tape recorder of audio grade.